Immature cells, so called stem cells, progenitor cells or precursor cells, with the potential to develop into different types of mature cells, is the most promising strategy for restoring injured tissue.
Stem cells (used in this text to denominate all types of immature, multipotent cells) can be isolated from different sources. Embryonic stem cells are derived from surplus fertilized egg cells, fetal stem cells are isolated from embryonic/fetal tissue, adult stem cells are derived from adult tissue of different types, and tumors are used for cancer stem cells. These self-renewing cells are kept in cell culture systems and expanded, to provide a source for different experimental and clinical purposes. In addition to the culturing of stem cells, many mature cells also undergo cell division and can also be expanded in cultures maintained the same way as stem cells.
Cells are grown under conditions, which favor their survival and growth, either as adherent cultures (i.e. all cells adhere to the bottom of the cell culture flask or dish) or as free-floating cell aggregates. With time, the number of cells in each culture flask increases and eventually the cells have to be split into more cell culture flasks. Splitting the cells usually involves separating the cells into a homogenous suspension of single cells (dissociation). Dissociation is done mechanically by grinding, or by adding proteolytic enzymes. Alternatively the cell aggregates can be cut into smaller pieces without dissociating them. This method involves cutting the tissue with razor blades or scissors, which is very time consuming and prone to contamination.
Minimizing cell death throughout the entire process is a prerequisite for successful expansion of cells in culture. Dissociation always leads to some cell death, probably as a combined effect of the rupture of cell membranes during the dissociation and the demise of cells after re-seeding them as single cells. Depending on the strength of the bonds between cells, different cell types may be more prone to damage during dissociation. Optimizing this process is therefore important. Adding enzymes for the dissociation process also introduces a risk of contamination as well as the problem that low concentrations of enzymes can remain in cells used for cell therapy. In conclusion, it would be beneficial to develop methods that enhance the survival and expansion of cell cultures, while eliminating the use of exogenous enzymes.
There are other applications for which reproducible cutting or slicing of biological tissue is an important part of the procedure. So called organotypic cultures are established by using thin slices of tissue, typically 200-400 μm. These slices are maintained in culture systems to preserve the cellular organization of the organ which was used. These slices or prisms of tissue are also used for acute in vitro experiments, for example studying the change in metabolism upon exposing the tissue to a drug. The last few years we have also seen tissue slices being used in so called high-content screening, test systems utilizing large-scale testing of drugs and substances in complex cell systems such as organotypic cultures.